GB2120791A

GB2120791A - Microwave gas-liquid void fraction meter

Info

Publication number: GB2120791A
Application number: GB08314044A
Authority: GB
Inventors: Jabez Whelpton
Original assignee: Atomic Energy of Canada Ltd AECL
Current assignee: Atomic Energy of Canada Ltd AECL
Priority date: 1982-05-27
Filing date: 1983-05-20
Publication date: 1983-12-07
Also published as: DE3316328A1; JPS58215542A; GB8314044D0; DK235183D0; IT8321023A0; IT1234922B; DK235183A; FR2527774A1

Abstract

A gas-liquid void fraction meter for measuring the ratio of gas to liquid flowing in a pipe comprises a structure (14, 15) defining a microwave cavity positioned on and encircling the pipe (10) such that the pipe passes centrally through the cavity, a first set of probe structures (12) positioned in the cavity, electrical power supply means connected to said probe structures for exciting the cavity at microwave frequencies and in a resonant mode such that the electrical field vector is parallel to the flow in the pipe (10). A second set of probe structures (12) is positioned in the cavity and electrical frequency measuring means are connected to the second set of probes to measure the frequency of the resonant cavity, said frequency being related to the mixture dielectric permitivity and thus the ratio of the gas to liquid flowing in the pipe. <IMAGE>

Description

SPECIFICATION Microwave gas-liquid void fraction meter This invention relates to a microwave two-phase void fraction meter and more especialty to a resonant microwave cavity apparatus for measuring the ratio ofwatertosteamflowing in a pipe.

One ofthefundamental quantities required to describe any two-phase flow is void fraction. The actual measurement may take several forms such as local, chordal average or cross-sectional average values. The type of fraction measurement used and its success depends on the application. Commonly applied local measurement methods include constanttemperature anemometry,fibre optic probe, isokinetic sampling probe, conductance probes and microthermocouples. Chordal average methods usually employy, X-ray or neutron beam attenuation measurements. Cross-sectional average orvolume average values may be measured by quick-closing valves, capacitance gauges, rotating electric field conductance gauges and neutron scattering techniques.

It is an object ofthe present invention to provide a gas-liquid void fraction meter that enables the ratio of liquid to gas flowing in a pipe-lineto be determined without disturbing the ratio or interrupting the flow.

The present invention provides a two-phase void fraction meter for measuring the ratio of one phase to another in a mixture flowing in a pipe, the meter comprising: a) a structure defining a microwave cavity posi tioned on and encircling the pipe such that the pipe passes centrally th rough the cavity, b) a first set of probe structures positioned in the cavity, c) electrical power supply means connected to said probe structures for exciting the cavity at micro wavefrequenciesand in a resonant mode such thatthe electrical field vector is parallel to the flow in the pipe, d) a second set of probe structures positioned in the cavity and e) electrical frequency measuring means connected to the second set of probes to measure the frequency of the resonant cavity, said frequency being related to the mixture dielectric permitivity and thus the ratio ofthe one phase to the other flowing in the pipe.

An RF cavity has been used for hydrogen density measurements (see Wenger N.C., Smetova J., "Hydrogen Density Measurements Using an Open-Ended Microwave Cavity" IEEETrans. on Inst, and Meas.

Vol. IM-21 No.2, May 1972) and the use of microwave devices forvarious other purposes is also known (see U.S. Patents 2,792,548; 3,501,692; 3,612,996; 3,818,333 and 4,042,879.

In the drawings which illustrate embodiments of the invention, Figure lisa cross-section schematic side view of the void gauge, Figure 2 is a cross-sectional schematic front view of the gauge, Figure 3 is a cross-sectional view ofthe gauge showing afirstform of probe arrangement, Figure 4 is a cross-sectional view of the gauge showing a second form of probe arrangement, Figure 5 is across-sectional view of a two phase, three componentflow dual RFsensing system, and Figure 6shows a typical excitation and read-out arrangement.

Referring to figures 1 and 2 tube 10 carrying the two-phase flow to be measured passes centrally through a cylindrical cavity formed by housing 11.

This cavity is excited in the TM010 mode by means of antennae 12 positioned as shown inside the housing 11. A cavity having a prism (triangular), ellipsoid, or rectangular cross-section may also be used and this would be exicted in the TE011 ore011 ore 101 mode.

Both arrangements are essentiallythe same. The elctricfields are parallel to the centrai tube 10 with the maximum strength of the field where the tube is located. The field is excited using small coupling loops (antennae) 12 about .1 wavelengths (ofthe exciting frequency) in area and these excite a magnetic field circumferential to the central tube axis.

In exciting the cavity the loops or probes are positioned sothat only the fundamental mode is excited. The response time is made faster by using multiple exciting loops excited in phase and with equal power. The receiving loops (which might be 2 ofthe 4 loops) are placed with respect to the exciting loops so that power transfer occurs in the described mode. This reduces the spurious excitation and enables the desired change in resonant frequency to be traced as the change in resonance occurs with different void fraction ratios.

Figure 3 is a cross-section of a working device with the microwave cavity structure formed by tube 14 and end plates 15 encircling pipe 10through which the material being measured flows. Microwave energy penetrates into the pipe through ceramic sleeve 13.

The antenna or probe 12 is merely a loop of metal grounded on one side with the inputloutput occurring on the other end. This probe is mounted by means of plates 16 and 17 on the sides of the cavity structure and the power input/output is taken via fitting 18.

Figure 4 is a transverse cross-section showing a differenttype of probe 12 mounting. The probe is mounted in antenna holder 19 and input/output power is taken through fitting 20 by connector 21 to output receptacle 22.

Figure 5 shows apparatus for measuring a two phasethreecomponentflowe.g. gasvapor/gas condensate/liquid oil orwater. A main RF cavity 31 with exciting and pick-up, probes 32 is positioned as shown aroung pipe 30 carrying the flow. A re-entrant branch line 40 carries liquid onlyfromthe main line through secondary cavity 41 having probes 42. The main cavity measures the ratio ofvaporto liquid and the secondary cavity measures the ratio of gas condensate to the oil or water.

Figure 6 is a typical power supply and readout arrangement made up of excitation power input source 50, signal splitter 51, output signal splitter 52 and readout device 53.

The power in is a swept sine wave of 10 dBm (referenced to 1 MW level) and the cavity at resonance generates a signal of approximately -30 dBm. The input signal may be applied to one or more coupling loops in parallel) orthrough a 90 hybrid.

This is a signal splitterwhich generates two outputs (one phase shifted 90C relative to the other) from one input. The output loops are connected through a similar device or alternately they may be connected in parallel. The input/output configuration and total numberofantennae or coupling loops is not critical except that there has to be at least one input and one output loop. Thetotal number of antennae must, therefore be two or greater. The cavity may have a single ceramic tube spanning it, oran alternate design where the metal sleeves protrude up to 20% of the cavity length into the cavity.

In each ofthe embodiments shown in the drawings, theRFcavitysurrounding a short section of pipe constitutes a field device whose resonant frequency depends on the dielectric permitivity ofthe gas-liquid mixture (or, in the case of cavity 41 in Fig. 5, the condensate-liquid mixture) flowing in the pipe. By measuring the resonant frequency, the ratio in which the two phases (gas-liquid orcondensate-liquid) are present in the mixture can be determined.

Claims

1. Atwo-phasevoid fraction meterformeasuring the ratio of one phase to another in a mixture flowing in a pipe, the meter comprising: a) a structure defining a microwave cavity posi tioned on and encircling the pipe such that the pipe passes centrally th rough the cavity, b) a first set of probe structures positioned in the cavity, c) electrical power supply means connected to said probe structures for exciting the cavity at micro wave frequencies and in a resonant mode such that the electrical field vector is parallel to the flow in the pipe, d) a second set of probe structures positioned in the cavity and e) electrical frequency measuring means connected to the second set of probes to measure the frequency of the resonant cavity, said frequency being related to the mixture dielectric permitivity and thus the ratio of the one phase to the other flowing in the pipe.

2. Atwo-phasevoid fraction meter as in claim 1 wherein the number of antenna probe structures is four positioned orthogonally around the cavity struc- ture.

3. Atwo-phase void fraction meter substantially as described herein with reference to, and as shown in, Figs. 1 and 2 or Fig. 3 or Fig. 4 or Fig. 5 ofthe accompanying drawings.